#include <bits/stdc++.h>

using namespace std;

int main(void) {
  int n, k;
  cin >> n >> k;

  if (k <= 0 || k > n) {
    cout << 0 << endl;
  } else if (n & 1) {
    cout << n - (2 * k - 1 == n ? 1 : 2) << endl;
  } else {
    cout << n - 2 << endl;
  }

  return 0;
}